Electrostatic precipitators have, for many years, served to remove suspended particles from streams of air or other gas. A particularly important application for these precipitators lies in the removal of fly ash from suspension in the gases emitted by large coal burning furnaces, such as those used in plants which generate electric power. The furnace gas passes through the precipitator before entering the smoke stack. The entire precipitator is composed of a number of sections, each of which has its own high voltage DC power supply. The circuit of a power supply may include a single-phase AC station power line, a circuit-breaker, silicon controlled rectifiers (SCRs) in inverse-parallel connection, a current-limiting reactor (CLR), and a transformer for producing a high AC secondary voltage which is electrically isolated from the station power line. Solid-state rectifiers may be used to convert the AC secondary voltage to unfiltered DC. The transformer and the rectifier are customarily packaged together and referred to as a Transformer-Rectifier Set (T/R Set). A section of the precipitator may be visualized as a duct containing vertically positioned metal plates in parallel arrangement. Electrodes, such as wires, which are connected together and to the high voltage output of the T/R Set, extend from a supporting structure downward between the plates, which are grounded.
In operation, the precipitator section must cause particles in the gas stream to move to and build up on the plates. This is accomplished by maintaining a DC voltage sufficient to produce a flow of ions from the electrodes to the plates. Automatic control of the production of ions, or corona, is essential to the proper functioning of the precipitator. It is also necessary to provide means for automatically controlling the removal of trapped particles. The matters of concern here are limited to the provision of means for automatically controlling the power input to the T/R Set of each precipitator section in a manner to improve its ability to prevent the escape of particulate matter and to use electrical energy efficiently.
In practice, the high voltage DC output of a T/R Set has been controlled largely through use of feedback signals produced by sparking between any of the HV electrodes and an adjacent grounded plate. Automatic means, in response to a spark signal, cut back the output of the T/R Set momentarily. The precipitator voltage was then gradually increased until another spark discharge occurred. In effect, the precipitator voltage was allowed to hunt between the voltages which produced sparking and substantially lower voltages. This mode of operation provided a control over spark rate which migh have a frequency as high as two sparks per second.
It is appropriate to explain here the function of the current-limiting reactor CLR in the primary circuit of the T/R Set, see FIG. 4. The inductance of this reactor should be such as to suit the normal range of load requirements of the precipitator section fed by the T/R Set. If a T/R Set is oversize with respect to its load, it may become necessary to increase reactor inductance to obtain proper performance of the T/R Set. The reactor serves these functions: It limits the rate of rise and the maximum primary current of the T/R Set during normal operation and in the event of a breakdown in either the power supply or in the precipitator. It stores energy while line current is increasing and returns this energy to the circuit while the line current is decreasing.
If the voltage of a precipitator were increased from zero to just below the point where corona develops, the precipitator would act as a capacitor, drawing charging current and storing energy (1/2 CV.sup.2 joules). As voltage is raised above the value where corona begins, the flow of ions from the HV electrodes to the plates, accelerates rapidly until spitting and sparking occur. Spitting, as here defined, is a low energy spark which causes a small drop in the precipitator's voltage and stored energy. A spark, as here defined, is a substantially heavier discharge which dissipates a larger portion of the energy capacitively stored in the precipitator. The precipitator voltage drops immediately to a lower value at which the spark discharge may end. The T/R Set is required to replace the loss of energy caused by the discharge and then to produce corona. Sparks may initiate power arcs that continue until the single phase AC supply to the T/R Set is interrupted. Frequent sparking is objectionable because each spark is followed by an interval during which the precipitator is inoperative, and then operative at reduced efficiency. A spark which develops into a power arc is more objectionable. Arcing may cause corona electrodes to break and short circuit the precipitator. A control system for the T/R Set capable of responding to spits can materially reduce the frequency of sparking and arcing. The precipitator's efficiency, in respect to trapping of particles and also in consumption of power, is then increased, and shut-downs for repairs become less frequent.
Reduction of sparking and arcing in all sections of a complete precipitator will materially reduce the production of transients which in quantity could have an adverse effect on a precipitator's overall performance.